Colonies have ground to space weapons, (Think Star Wars: The Empire Strikes Back) so it makes sense that a 400Mu planet would take a lot of shelling to kill all the resistance. Before attacking (if you know colony size) just multiply the Population (just the 3 digits) by 18 and then divide that by the ATK of one of your ships. That'll tell you how many ships of that kind you need to take the planet. Don't forget to add in Orbitals and other defending ships, if they have them.